Proposed studies will examine the nature of immunosuppression of humoral and cellular responses in experimental Chagas' disease in mice. Attempts will be made to determine the cellular basis of suppression, characteristics and action of a soluble suppressor substance induced by Trypanosoma cruzi and the significance of immunosuppression on the course of the disease. Preliminary evidence suggests that immunosuppression is an H-2 related phenomenon and experiments on the immunogenetics of suppressor substance-target cell(s) interaction are planned. It is suggested that abrogation of suppression by treatment of mice with xeno- and alloantiserum against the suppressor substance and other procedures be attempted to determine the ability of normally susceptible mice, when rendered responsive (unsuppressed), to resist infections. Other experimental manipulations are described to study the implications of transiently induced suppression observed in mice which have been experimentally immunized with parasite extracts or vaccinated with mitomycin C attenuated trypanosomes.